What is global _start in assembly language?

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global directive is NASM specific. It is for exporting symbols in your code to where it points in the object code generated. Here you mark _start symbol global so its name is added in the object code (a.o). The linker (ld) can read that symbol in the object code and its value so it knows where to mark as an entry point in the output executable. When you run the executable it starts at where marked as _start in the code.

If a global directive missing for a symbol, that symbol will not be placed in the object code’s export table so linker has no way of knowing about the symbol.

If you want to use a different entry point name other than _start (which is the default), you can specify -e parameter to ld like:

ld -e my_entry_point -o output_filename object_filename

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